The invention relates to electronic devices, and, more particularly, to electronic temperature sensors and methods of temperature sensing.
Many manufacturing processes, such as integrated circuit fabrication, require accurate temperature sensing in real time as part of process control. Indeed, in integrated circuit fabrication, thermally activated steps demand precise wafer temperature control. The current trend in integrated circuit fabrication to single wafer processing instead of batch processing permits in situ individual wafer temperature monitoring. However, such wafer temperature monitoring should be by a direct but noncontact approach.
Current integrated circuit fabrication generally employs thermocouples contacting the backside of a wafer for temperature sensing. However, poor contact implies a loss of accuracy. Furthermore, the frontside surface temperature determines processing behavior, not the wafer backside surface temperature. But frontside contact leads to surface damage and perturbs chemical processes. Thermocouples consist of metals which may be damaged by the harsh chemical environment during wafer processing and also may contribute contaminates to the processing.
Pyrometery offers a direct, noncontact measurement of the emissive radiant power from a surface (black body radiation). However, the emissivity (absorptance) of the surface provides a generally unknown factor and prevents the simple computation of the surface temperature from the emissive radiant power. Indeed, the emissivity of wafer material(s) will depend upon the structure. Typically, wafers will include multiple, somewhat transparent surface layers of various materials.
Hansen et al, 28 Appl. Optics 1885 (1989) measured the emissivity of certain transition metals (which are opaque) by ellipsometry with a HeNe laser source (633 nm) and also measured the radiance at the same wavelength to compute the temperature.
Another approach relies on the temperature dependence of the (complex) dielectric constant of a material and an ellipsometric determination of the dielectric constant to deduce the temperature. Ellipsometry does provide a direct, noncontact measurement, but the weak temperature dependence of the dielectric constant for wafer materials of interest such as silicon implies insufficient accuracy. See Ibrahim et al, 9 J. Vac. Sci. Tech. 1259 (1972) and Tomita et al, 25 Japn. J. Appl. Phys. L925 (1986).